For reasons relating to the nature of the sensor and the measurement intended to be carried out, for example the measurement of an acceleration of an object liable to experience large stresses, it is sometimes necessary to mount the cell in such a way that the vibrations of the object carrying the sensor, or the shocks experienced by this object, do not affect the measurement or the structure of the cell. This is because an accelerometer is typically very sensitive to vibrations and shocks, and it would deliver an electrical signal which is difficult to process if a parasitic signal due to vibrations of the object whose acceleration is to be measured were superimposed on the acceleration measurement per se. On the other hand, vibrations and shocks could damage the sensor, which would be even more detrimental. The measurement of acceleration is not the only case in which vibrations and shocks are causes of malfunctions or measurement difficulties, but it does constitute a typical case to which the invention is particularly applicable. Sensors of other physical quantities, produced by micromachining, may typically be affected.
The cell is then mounted in a package with the interposition of a shock and vibration damping element. For example, the cell is wedged in the package by elastomeric wedging blocks whose mechanical damping properties are suitable for the shocks and vibrations to be filtered, and the cell is not in direct physical contact with the package. However, it is necessary to transmit supply voltages or electrical signals between the cell and the electronics board associated with it, which board is also mounted in the package. Care therefore needs to be taken so that the electrical connections do not transmit vibrations and shocks, which the damping elements are intended to absorb, to the cell owing to their stiffnesses.
One possible method of electrical connection between the microsensor cell and the electronics board is represented in section in FIG. 1 and as a plan view in FIG. 2. The cell 10, with its electrical connection pins 12, is contained in a package 20 closed by a cover 22; it is held in place by damping blocks 24 (usually made of elastomer) which support it while absorbing the shocks and vibrations transmitted by the package. The package may be fastened on an object on which a measurement is performed, in which case the fastening (not shown) may be carried out by any means.
The cell lies on the rear face side of an electronics board 30, which carries components 32 and printed conductors 34 on its front face; the board 30 is pierced by holes 36 in which the connection pins 12 of the cell engage freely (without physical contact) through the rear of the board; the ends of the pins are connected by soldered wires 14 to conductive terminals forming part of the printed conductors 34 of the front face of the board; the wires are soldered on the one hand to the end of the pin and on the other hand to a respective terminal associated with this pin; the wires 14 are not straight, but instead are curved so as to act as a spring having a small stiffness in all directions (a straight wire would have a high stiffness in the direction of this straight line).
In order to ensure sufficient electrical conductivity between the board and the pin (preferably less than one ohm), the wires are typically made of bare gold or aluminum, or copper insulated by a plastic sheath, or copper or silver coated with an insulating enamel, etc. The diameter of the conductive wire is typically 50 micrometers, and its length is a few millimeters. The wire must be preformed before or during the soldering operation in order to give it the curved shaped which ensures a low stiffness in all directions and, like the operation of holding the wire when soldering, this preforming operation is difficult to carry out.
It has been observed that the modules produced in this way are excessively sensitive to vibrations, which prevents correct measurement.
Another way of producing the connection between the sensor and the board may consist in using a flexible printed circuit layer, as is done in printers, camcorders, etc. However, these layers are generally not flexible enough because of the relatively rigid insulating plastic material on which the electrical connections rest.
There is therefore a need for a method of simple connection with very low stiffness between the cell and the electronics board of a sensor liable to be subjected to large vibration stresses. It is an object of the invention to provide a solution which at least partly improves the known solutions.